Problem Reading from XenStore in DomU

[Dagaen Golomb <dgolomb@seas.upenn.edu>]

Hi All,

I'm having an interesting issue. I am working on a project that
requires me to share memory between dom0 and domUs. I have this
successfully working using the grant table and the XenStore to
communicate grefs.

My issue is this. I have one domU running Ubuntu 12.04 with a default
3.8.x kernel that has no issue reading or writing from the XenStore.
My work also requires some kernel modifications, and we have made
these changes in the 4.1.0 kernel. In particular, we've only added a
simple hypercall. This modified kernel is what dom0 is running, on top
of Xen 4.7 rc1.

The guest I mentioned before with the default kernel can still read
and write the XenStore just fine when on Xen 4.7 rc1 and with dom0
running our kernel.

The issue I'm having is with another newly created guest (i.e., it is
not a copy of the working one, this is because I needed more space and
couldn't expand the original disk image). It is also running Ubuntu
12.04 but has been upgraded to our modified kernel. On this guest I
can write to the XenStore, and see that the writes were indeed
successful using xenstore-ls in dom0. However, when this same guest
attempts to read from the XenStore, it doesn't return an error code
but instead just blocks indefinitely. I've waiting many minutes to
make sure its not just blocking for a while, it appears like it will
block forever. The block is happening when I start the transaction.
I've also tried not using a transaction, in which case it blocks on
the read itself.

I have an inkling this may be something as simple as a configuration
issue, but I can't seem to find anything. Also, the fact that writes
work fine but reads do not is perplexing me.

Any help would be appreciated!

Regards,
Dagaen Golomb
Ph.D. Student, University of Pennsylvania

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